An MRI apparatus obtains echo signals necessary to reconstruction of a sheet of an image by repeatedly executing a pulse sequence (hereinafter, simply referred to as “sequence”) while varying an amount of phase encode. Accordingly, an imaging time is greatly affected by the number of repetition of the sequence. When an imaging is executed at a high speed, a multi-echo type sequence is ordinarily used to generate a plurality of echo signals by executing the sequence once. Further, a sequence whose repetition intervals are shortened to several to several tens of microseconds is used. However, these sequence may decrease an image contrast or distort an image. This is because the echo time that contributes to the image contract is different in the respective echo signals in the multi-echo type sequence. Otherwise, this is because when a phase varies differently between respective echo signals due to a different echo time, this phase variation appears as distortion of an image.
In contrast, a high speed imagig called a parallel imaging method is proposed to take a picture of a heart region such as a coronary artery because it is necessary to measure an image at a higher speed in this case. The parallel imaging method is a method of reducing an imaging time by reducing the number of repetition of a sequence by using a plurality of RF receiving coils (multiple RF receiving coil) and executing the sequence while thinning phase encodes at equal intervals. Ordinarily, when an image is constructed as it is using signals measured by thinning the phase encodes at equal intervals, aliasing occurs in the image. To cope with this problem, there are heretofore known a signal processing method (SENSE: Sensitivity Encoding for Fast MRI (Klass P. Pruessmann et. al, Magnetic Resonance in Medicine 42: pp 952-962 (1999)) and the like that eliminate the aliasing of the image by executing a matrix calculation based on the sensitivity distributions of the respective RF receiving coils. That is, the signals measured from the portions of an image having aliasing are superimposed with the signal components of different aliased regions. To cope with this problem, the aliasing portions of the image are unfolded by establishing simultaneous equations according to the correlation among the signals measured by the plurality of RF receiving coils, the sensitivity distributions of the respective RF receiving coils, and the signal components at a plurality of points on a measuring space, by solving the simultaneous equations by a matrix calculation, and by separately determining the signal components of the region where the aliasing portions overlap. In general, in the parallel imaging method, an imaging time can be shorted by the number of the RF receiving coils used in the matrix calculation. That is, in principle, phase encode matrixes as many as the number N of the RF receiving coils used in the matrix-calculation can be thinned, thereby the imaging time can be shortened to 1/N.
However, an image cannot be properly unfolded because the matrix calculation may be diverged depending on the positional relationship among the plurality of RF receiving coils used in the matrix calculation or a combination of the sensitivity distributions of the RF receiving coils. For example, there is no difference in the sensitivity distributions among the RF receiving coils used in the matrix calculation, the image may not be properly unfolded because the matrix calculation is diverged. Further, when the matrix calculation is executed while including the signal components of a low signal region such as a background and the like, an error may be increased by the affect of noise when aliasing portions are unfolded. As a result, the SN ratio of an image having been unfolded may be extremely deteriorated or spot artifacts may occur in an image.
Further, even if the positional relationship among the RF receiving coils used in the matrix calculation is optimum with respect to a specific direction, there is a possibility that artifacts and deterioration in image quality may occur in a resultant image when a measuring axis and a measuring slice section are varied or when an encode axis is varied.
On the other hand, according to the aliasing elimination method employing the matrix calculation described above, it is necessary to improve the accuracy of the sensitivity distributions of the respective RF receiving coils. To accurately calculate the sensitivity distributions of the respective RF receiving coils, the sensitivity distributions of the respective RF receiving coils are calculated using an image calculated by an entire body coil having a relatively uniform sensitivity distribution.
However, since there is an MRI apparatus that is not provided with the entire body coil, it is preferable that the aliasing portions of an image be accurately eliminated even in such a case. Further, when the number of channels of received signals is small, there is a case that images transformed from the sensitivity distributions of the respective RF receiving coils and images transformed from the sensitivity distribution of the entire body coil cannot be obtained simultaneously.